The present invention is directed to lead-acid batteries, and particularly to lightweight, high energy batteries having electrodes formed of a non-lead, lightweight conductive substrate covered by a corrosion prevention layer and a lead, outer layer.
Lead-acid batteries conventionally include a multiplicity of cells connected together in series. Each cell consists of a stack of alternating electrodes,namely cathodes and anodes. Often there is a separator between the electrodes whick absorbs and holds the electrolyte (generally sulfuric acid).
In the past, electrode plates have been formed primarily of lead castings, stampings, or an expanded mesh of lead or of a lead compound which provides the structural element to support the electrochemically active material of the electrode. When charged, the electrodes become positively or negatively charged, where the energy is stored, until used in whatever application the battery is put. The battery may also be recharged from time to time.
Lead has been predominately used as the plates such batteries for a long period of time. While lead is not particularly a good conductor of electricity, it is inherently corrosive resistant to the electrolytic acids. Other, more conductive metals are either too expensive to be used as the electrode for lead-acid batteries, or else they are quickly corroded during the charging action by the electrolytic acids. Therefore lead has remained as the predominant material. Lead is also very heavy, and in applications where weight is a factor, other alternatives have long been sought.
For example, in the aircraft industry, experts have calculated that the fuel cost of flying a commercial airliner is more than $3,000 per year per pound of weight flown. Therefore, if the airplane carries batteries having lead plates, considerable sums of money could be saved per plane if a lighter weight plate material could be found.
In previous attempts to replace lead as the predominant material, one approach has been to plate lead onto other more conductive metals or metal alloys such as aluminum and copper. Copper is sixteen times as conductive as lead and weighs only about 70% as much. Aluminum, on the other hand has a specific gravity of only 20%-25% of lead and approximately eight times the conductivity of lead. Obviously, from the standpoint of weight and conductivity, copper and aluminum are good candidates to replace lead as the substrate for electrodes. However both materials are very susceptible to corrosion in the presence of sulfuric acid, and cannot be used as the electrode in a lead acid battery if left unprotected. Either material can be used as the negative electrode, and copper has in the past when coated with lead. In previous attempts to use aluminum or copper as the primary structural element for the plates of a lead acid battery in the past, attempts have been made to plate lead coatings onto aluminum or copper substrates. The conventional manner for plating lead is from an aqueous solution. A problem arises when lead is plated directly on a substrate from an aqueous solution. For one reason or another, the coatings are porous, and the sulfuric acid will quickly penetrate the coatings and attack the aluminum or copper. In such instances, the copper and aluminum plates have not survived the charging operation.
The present invention is directed toward reducing the weight per unit mass of the battery by replacing a significant portion of the lead or lead alloy plates of the battery with a lighter weight, conductive material that is protected from the corrosive effects of the electrolytic acid by the combination of a corrosive prevention layer plated with lead or a lead alloy. Toward this end, then, the present invention utilizes a highly conductive non-lead, lighter weight substrate as the structural material for lead-acid battery electrode plates. The non-lead substrates may be aluminum, magnesium, copper, aluminum alloys, or aluminum/magnesium alloys, plastic coated with graphite, or other conductive structures. A substrate formed from any of these materials is significantly lighter than lead, having a specific gravity of no greater than 70% that of lead. The substrate may be formed as a sheet, a wire mesh, an expanded metal mesh, or a perforated sheet.
The non-lead substrate is coated with a relatively thin corrosion prevention layer of such materials as nickel, silver or gold that is resistant to the electrolytic acid to be used in the battery. As used herein, xe2x80x9ccorrosion preventionxe2x80x9d includes metals having a high resistance to corrosion and oxidation such as nickel, silver and gold. This corrosion prevention layer is plated onto the lightweight substrate by electroplating or an electro-less plating procedure.
Following plating of the corrosion prevention layer, a continuous outer layer of lead or a lead alloy may be applied. Because of the added protection provided by the corrosion prevention layer, the continuous outer layer need only be sufficiently thick to provide adequate life to the battery, as for example, 0.001 to 0.002 inches. As a result of applying the corrosion prevention layer, the outer layer of lead, lead compound, or other suitable outer layer of conductive corrosive resistant material may be applied by any of the conventional methods for coating such as dipping, molten salt bath deposition, electroplating by aqueous solution, or spray, vacuum, or plasma deposition. By first coating the lightweight conductive substrate with the relatively thin corrosion prevention layer of nickel, gold or silver, the porosity of the outer layer is of less concern. Further sealing of the outer conductive layer may provide an additional measure of protection to increase the life of the battery.
By constructing an electrode plate as described above, the energy to weight ratio, when compared to conventional lead plate cells, is in the range of 35-65 Watt-hours/kilogram (WH/kg) or more. That is to say, rather than an energy to weight ratio of approximately 30 WH/kg as in the case of conventional lead plate lead-acid batteries, the energy to weight ratio of batteries of the present invention may be in the range of about 35-65 WH/kg or more.
The electrode of the present invention are pasted with conventional negative or positive pastes and charged to create corresponding electrodes. These electrodes may be utilized in any number of battery cell configurations. Alternating positive and negative electrode plates are generally separated by a separator such as a thin glass mat (which absorbs and carries the electrolytic acids) and arranged in a conventional parallel plate arrangement.